The invention describes thick film getter compositions for use in controlling moisture, oxygen, and small organic molecule levels. Many packaged devices are susceptible to damage from undesired levels of moisture, oxygen, and small organic molecules and this is especially so in the case of various electronic, display, and light sensing or emitting devices. Examples of the types of damage caused or aggravated by the gas or condensed phase impurities include; hydration, oxidation, corrosion, and ion migration. In the case of FED (Field Emission Display) or SED (Surface Emission Display), the residue gasses may cause the undesirable modification of the emitter tip surface work function and degrade the performance. The concept of using a getter to control the atmosphere within packaged devices has been known for many years. Getters are substances that are added to a system or mixture to consume or deactivate traces of undesirable gas phase impurities including moisture.
One approach to minimizing the deleterious effects of gas phase impurities has involved the enclosure of sensitive devices with inorganic or organic barriers to physically separate the active materials from external atmospheric oxygen and moisture. This approach has had some success, but it does not always adequately address the problems caused by even those small amounts of gas phase impurities trapped within the enclosure or diffusing into the enclosure over time.
The use of getters for controlling gas phase impurities within a sealed enclosure for an electronic component or device is well known. These sealed enclosures are designed to protect sensitive electronic components and devices from gas phase impurities including moisture that come from outside environmental contaminants and/or internally created outgassing or desorption from various functional components. For example, a great many of display devices such as PDP (Plasma Display Panel), CRT (Cathode Ray Tube), FED (Field Emission Display), LCD (Liquid Crystal Display), LED (Light Emitting Diode) are highly sensitive and can only tolerate very low levels of gas phase impurities. At the same time, mass production of these devices dictates that the getter composition be easily processed while providing sufficient adhesion to any chosen substrate. The present invention provides a superior composition and process to accommodate these requirements.
Prior art getters and their use in electronic applications are described below. Typically, the getter materials are not screen-printable, and normally are comprised of a desiccant material (i.e., zeolite, silica gel, etc.) and a binder. The binder may be organic or inorganic. The following illustrates the specifics of the prior art.
U.S. Pat. No. 5,244,707 to Shores discloses a sealed enclosure of an electronic device which incorporates a coating or adhesive with desiccant properties. The coating or adhesive comprises a protonated aluminum silicate powder dispersed in polymer.
U.S. Pat. No. 5,591,379 to Shores teaches a composition of matter useful as a desiccant in a hermetic electronic device, comprising a powder dispersed in a binder, wherein said powder is selected from the group consisting of zeolite molecular sieves, activated alumina, silica gel, alkaline earth oxide, and alkali metal carbonate; said binder is a continuous matrix of porous glass or porous ceramic; and said powder to binder volume ratio is 0.001-2. The glasses disclosed for use as a binder must be made porous by creating channels for water vapor to penetrate. This may be done by various techniques known in the art, such as the use of blowing agents, fast evaporation of water or other gases during formation, fast decomposition of organometallic polymers and low temperature or incomplete sintering.
U.S. Pat. No. 1,626,682 to MacRae discloses a multi-pixel flat panel display which includes spaced apart first and second electrodes, with a patterned solid material layer in contact with at least one of the electrodes, and preferably placed between the two electrodes. The patterned layer (the web) includes a multiplicity of apertures, with at least one aperture associated with any given pixel. In the aperture is disposed a quantity of a second material, exemplarily, a phosphor in the case of a flat panel field emission display, or a color filter material in the case of a liquid crystal display. The web can facilitate second material deposition by means of screen-printing. The web also can facilitate provision of spacer structure between two electrodes, and can include getter or hygroscopic material.
U.S. Pat. No. 5,401,706 to Fischer teaches a process for making a desiccant-coated substrate capable of being used at temperatures above 150 degrees Fahrenheit, the desiccant being in the form of particles and the particles having pores and being adhered to the substrate by a binder, the coated substrate being sufficiently flexible and the coating having sufficient adherence to the substrate so that the coated substrate can be formed into corrugated shapes, the desiccant particles in the coated substrate having at least 60% of their original adsorption capacity and the binder having good breathability; said process comprising the steps: (a) forming an aqueous suspension comprising particles of one or more desiccants, a water-based organic binder, a suspending agent to help maintain the desiccant particles in suspension, and an organic pore-clearing agent at least some of which enters at least some of the pores of the desiccant particles; (b) depositing the suspension on the substrate; and (c) causing the binder of the deposited suspension to set so that the deposited desiccant particles adhere to the substrate and causing at least some of the pore-clearing agent to leave the pores of the desiccant particles to prevent the binder from occluding at least some of the pores of the adhered desiccant particles, thereby to form a desiccant-coated substrate capable of being used at temperatures above 150 degrees Fahrenheit and of sufficient flexibility and having a coating having sufficient adherence to the substrate so that the desiccant-coated substrate can be formed into corrugated shapes and in which the desiccant particles in the coated substrate have at least 60% of their original adsorption capacity and in which the binder has good breathability.
Commonly assigned, U.S. patent application Ser. No. 10/712,670 to Cho et al provides a screen-printable thick film getter composition comprising glass which may be used to promote densification at relatively low sintering temperatures of 400-650° C. and to provide increased adhesion between the thick film and the substrate. Furthermore, the composition of Cho et al may be utilized in display devices which often require humidity control to levels below about 1000 ppm and in specific cases below 100 ppm. However, the thick film compositions of Cho et al may become unstable during generation of the thick film composition itself, due to the absorption of moisture from the atmosphere during processing. This absorption of moisture may lead to the generation of heat and possible safety concerns during composition processing. Furthermore, this absorption of moisture during thick film composition formation may modify the composition rheology, which is problematic during application.
The present invention provides a screen-printable thick film getter composition comprising glass which may be used to promote densification at low densification temperatures in the range of 400-650° C. and to provide increased adhesion between the thick film and the substrate. Another key ingredient in the getter composition is a pre-hydrated desiccant wherein the process of pre-hydration provides stability of (1) the desiccant under normal temperature/humidity environment and (2) the rheological behavior of the getter composition. Furthermore, the composition of the present invention may be utilized in electronic and photonic devices which often require humidity control to levels below about 1000 ppm and in specific cases below 100 ppm.